Summary of Significant Accounting Policies - Prior Period Adjustment (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities	$ 11,260	$ 16,974
Net cash used for investing activities	(103,118)	(33,968)
Net cash provided by (used in) financing activities	$ 84,360	13,527
Activities as originally presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by (used in) financing activities		13,816
Adjustment for shares tendered for tax withholding
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		289
Net cash provided by (used in) financing activities		(289)
Accounting Standards Update 2016-15
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		16,974
Net cash used for investing activities		(33,968)
Accounting Standards Update 2016-15 | Activities as originally presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		19,886
Net cash used for investing activities		(37,057)
Accounting Standards Update 2016-15 | Adjustment Relating To Restricted Cash [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		(148)
Net cash used for investing activities		36
Accounting Standards Update 2016-15 | Adjustment relating to interest income presentation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		(3,053)
Net cash used for investing activities		$ 3,053